Blasting method for beneficiating minerals

ABSTRACT

A blasting method for beneficiating minerals comprising mining of a geological body ( 10 ) including a value portion ( 20 ) having at least a threshold content of a mineral to be extracted and a waste portion ( 30 ) relatively lean in content of a mineral to be extracted wherein mining comprises controlled blasting of the geological body ( 10 ) such that the value portion ( 20 ) fragments differently than the waste portion ( 30 ), the difference in fragmentation between the value portion ( 20 ) and the waste portion ( 30 ) enabling separation of a mineral stream concentrated in the mineral to be extracted from the waste portion ( 30 ).

The present invention relates to a blasting method for beneficiating minerals, for example beneficiating ores to increase metal grade prior to mineral processing.

Energy comes at a great cost to our world and societies so it should be used wisely and optimally. There is a need for industry, including the minerals industry, to respond proactively to calls for energy conservation.

The major impediments to more efficient energy use in the minerals sector are driven by the scale and complexity of mineral resource extraction. Energy use on mining operations is complex and there is much scope for optimizing extraction energy. High energy consumption in mineral production arises primarily as follows:

-   -   Moving huge tonnages of ore and waste material. In most precious         and base metal operations more than 96% of the material moved is         uneconomic.     -   Ore comminution to liberate the micron scale valuable minerals.         Fine grinding is energy intensive and has a low efficiency in         creating new mineral surface area for liberation of metals.     -   Increasingly prevalent lower grade deposits require         significantly more energy to achieve equivalent metal liberation         compared to historical higher grade deposits.

The mine to mill approach to integrated mine and SAG mill optimization that came out of the JKMRC demonstrated that throughput gains of up to 20% could be obtained through a closer match of blasted run-of-mine ore to the feed best treated by a SAG mill (Various, 1998, Proceedings Mine to Mill, 11-14 October, Brisbane, 248p, Morell and Valery, Influence of feed size on AG/SAG mill performance, in Proceedings International Autogenous and Semi Autogenous Grinding Technology 2001 (Eds. Barratt et al.), vol 1, pp 203-214, University of British Columbia: Vancouver); Lam et al., Maximising SAG mill throughput at Porgera gold mine by optimizing blast fragmentation, in Proceedings International Autogenous and Semi-Autogenous Grinding Technology 2001 (Eds. Barratt et al.), vol 1, pp 271-288 (University of British Columbia: Vancouver); Eloranta, Optimised iron ore blast designs for SAG/AG mills, op cit, pp 262-270; Scott, Morell and Clark, Tracking and quantifying value from ‘mine to mill’ improvement, Value Tracking Symposium, 2002, pp 77-84; and Bye, The strategic and tactical value of a 3D geotechnical model for mining optimization, Anglo Platinum, Sandsloot Open Pit, Journal South African Institute of Mining and Metallurgy, 106 (2), pp 97-104).

There are new drivers since the inception of mine-to-mill that bring a new imperative to the ongoing quest for optimal operation:

-   -   Energy—the world at large is acutely aware of the cost and         penalty of excessive energy use.     -   Water—the competition for this resource and absolute constraints         on its availability.     -   Massive low-grade orebodies—leading to vastly increased waste         and often these ores are more competent and complex than older         orebodies.     -   Increasing demand for natural resources.

The consequence of the increase in demand and the drop-off in grade is that even a saving of 10 to 20% of energy usage in terms of kWh/t of ore does not offset the massive increase in total energy usage per unit of product required in the future. Based on the trends of mineral usage published in the report on Australian minerals usage (Australian Bureau of Agricultural and Resource Economics (ABARE), Minerals and Energy [online], volume 14, no 4, 2007. Available http://www.abareeconomics.com/publications.html/energy/energy 07/me nov 07 listing.pdf>), the average rise in energy consumption is about three per cent per annum. It is estimated that ore grade is dropping at about one percent per annum. Based on these figures, to achieve the oft-stated company targets of 10 to 20% saving in absolute energy (RioTinto, 2008, BHP Billiton, 2009; Anglo, 2009), specific energy usage (kWh/t ore treated) needs to drop below 40% of the usage in 2000 by 2020 and 20% by 2050. Applying current technology will not achieve anything near this objective.

Comminution, the size reduction process preceding metallurgical processing, requires very significant energy consumption. Overall comminution energy usage in the mining and concentrator stages of the process is as tabulated in Table 1.

TABLE 1 Approximate Distribution of Energy Usage Feed Size Product Energy Stage (mm) (mm) (% of process) Mining >1000 <300 1 (size reduction only) Crushing 300 20 10 Milling 20 0.1 89 Subtotal 100 Fine grinding 0.1 0008 100 (extra)

The table confirms that major opportunities for energy reduction are in milling. Due to the very high energy cost of fine grinding, if it is incorporated in the flow sheet, it must be used judiciously. However, in the case of finely disseminated ores—whatever the minerals of interest (base metals, iron, tantalum or other)—fine grinding is typically required as part of the extraction process.

Following comminution, concentration steps are typically required, sometimes being conducted distant from the mine (raising the issue of transportation costs), to upgrade metal content in the feed material to concentration to a level sufficient to enable economic metal extraction. Concentration involves separation of metal rich ore particles from waste or gangue particles through control over the surface properties of the feed material to concentration in concentrators. Concentrators may also have a large “footprint” taking up a significant amount of space. Such concentration steps, which may involve flotation, density separation and other operations, also add capital and operating expense to processing of a mineral resource. For example, flotation is a common operation for the beneficiation of base metal sulphide ores, such as copper, zinc and lead bearing ores. Again, however, where an ore is finely disseminated and fine grinding has been required, difficulties may arise during flotation operations due to the changes in surface activity of the ore particles during fine grinding. If concentration could be simplified, this would be advantageous for mineral resource economics.

It is an object of the present invention to provide a method for beneficiating minerals which enables reduction in energy consumption and other capital and operating costs in a mining operation.

It is a further object of the invention to provide a method for beneficiating minerals that increases economically viable ore reserves.

With this object in view, the present invention provides a blasting method for beneficiating minerals comprising mining of a geological body including a value portion having at least a threshold content of a mineral to be extracted and a waste portion relatively lean in content of a mineral to be extracted wherein mining comprises controlled blasting of the geological body such that the value portion fragments differently than the waste portion, the difference in fragmentation between the value portion and the waste portion enabling separation of a mineral stream concentrated in the mineral to be extracted from the waste portion.

Such selective fragmentation of the geological body differs from conventional blasting which is designed to blast an entire block of the geological body at a given energy to produce a top size that can be processed through a primary crushing step, further comminution steps and later in concentrators. For example, such a scheme would be commonplace in the treatment of base and precious metal ores.

The geological body typically is, or typically includes, an orebody or ore block containing valuable metal containing minerals in association with waste or gangue minerals. Gangue minerals typically have negligible economic value, that is they have very low grade or content of the valuable metal containing minerals. The geological body may comprise value portion in the form of “rich” zones which are rich in valuable metal containing minerals (highest grade zones of the geological body), and waste portion in the form of gangue zones (zones having insufficient grade for economic extraction. The geological body also typically comprises transition zones in which the grade or content of valuable metal containing minerals is intermediate the grade of the gangue zones and rich zones. Such transition zones are comprised within the term “value portion” used in this specification. Indeed, the efficient extraction of minerals from the transition zones may make the difference between economic and uneconomic extraction of minerals from the geological body. The respective zones are identified by mineralogical investigation of the geological body, this investigation involving assay to determine grade at particular locations within the geological body. Blasting is advantageously controlled to achieve the highest degree of fragmentation in the rich zones of the geological body. Material recovered from the rich zones of the geological body during mining would therefore have the finest fragment or particle size distribution; that is finer fragment distribution than from transition or gangue zones.

The metal containing minerals may be sulphide or oxide minerals of base metals (including copper, lead, zinc) and/or precious metals (including gold, silver and platinum group metals). Metal containing minerals may include iron ores. The beneficiation method may enable reduced comminution energy usage. As the controlled blasting of the method enables a size basis for concentration of the mineral to be extracted, and thereby increase of the effective metal content of the ore block, the beneficiation operations required following mining may be simplified and energy requirements reduced. The pre-concentration and size reduction that can be achieved by efficient controlled blasting will also have a major benefit on the down stream process generally.

Separation of the material relatively more concentrated in the mineral to be extracted, after mining, advantageously proceeds through physical separation. Separation on the basis of fragment size is particularly advantageous. In one scenario, though more complex scenarios may be envisaged, blasting may result in production of a “fine” stream and a “coarse stream”. The “fine” material has a finer size distribution than the coarse stream and blasting may be controlled to enable focused generation of fines in value portion(s) of the geological body, it further being observed that the value portion(s) have at least a threshold content or grade of mineral/metal to be extracted and likely significantly higher. Such threshold content or grade is the minimum enabling economically viable extraction of the mineral from the geological body or deposit. As to economic viability, use of the method may also change what is defined as economic ore through blast beneficiation. This would significantly increase the available ore reserves of mining operations by upgrading marginally economic ore and/or converting low grade ‘stockpile’ material into mill feed grade material.

The fine ore material has a higher value mineral or metal content than mineral or metal content in material having the coarse fragment or particle size distribution. Advantageously, the ore material highest in mineral or metal content has the finest fragment or particle size distribution. The coarse or waste stream may then be separated from the fine stream by screening and other operations suitable for separation by particle size. Screen size is selected to split coarse and fine streams at a cut particle size which optimises the separation of coarse stream from fine stream, optimising metal recovery and minimising the volume of waste directed to extractive metallurgical operations. This cut particle size may be determined from the particle size distributions for fine and coarse material and selected to optimise recovery of the mineral(s) to be extracted.

Separation of the material stream relatively more concentrated in the mineral to be extracted may advantageously be achieved by sorting on the basis of a physical or chemical property of mineral(s) contained in the value stream. Dry sorting, which avoids requirement for use of water, is preferred for beneficiation by sorting. Optical, electrical and/or magnetic properties of the minerals to be separated may be used as a basis for separation. Where optical sensing is used as a basis for separation visible light, X-Ray light or UV light may be used as a basis for separation.

However, it is logical that other physical separation steps may be used. Use of further concentration steps, such as flotation, is not precluded particularly for complex ore bodies, for example containing polymetallic ores requiring to be separated. It is expected that such further separation steps may be simplified by use of the controlled blasting process.

Although the above description implies that a single coarse stream and single fine stream will be generated by controlled blasting in accordance with the method, this is solely for purposes of illustration and outcomes from the method may be more complex. That is, an ore may be sorted to produce a number of streams or fractions using the method, these streams then being further separated or fractionated to produce a plurality of concentrate streams enriched to varying degree in the same or different minerals. The mineral streams may then be directed to further extraction steps which may be selected with reference to mineral or metal grade of each stream. Hydrometallurgical or pyrometallurgical processes may be used. For example, in the case of a gold bearing ore, the fines fraction may be treated by cyanidation. A coarse fraction having a lower gold content, perhaps bound within an iron matrix—perhaps of pyrite or arsenopyrite—may be treated by heap or dump leaching. Alternatively, if there is no economic incentive for further treatment, the coarse fraction may be rejected.

Blasting may be controlled to achieve such selective fragmentation behaviour, and beneficiation as above described, through a number of methodologies. Characterization and/or evaluation of the geological body, for example an ore block, is required as a first step. Exploration and sampling, involving identification of rock properties and downhole assay at different intervals by reverse circulation drilling, allow a three dimensional (“3D”) model of the geological body to be generated using various computer simulation packages. The geological characterization step may particularly focus the method for use where value portions are isolated or makeup small tonnages that are uneconomic to mine selectively with large mining equipment. This is often the case at the periphery of geological bodies. Characterisation and evaluation also requires the geo-mechanical characteristics of the geological body to be identified. This needs rock material properties to be identified, for example by use of geophysical probes.

Characterisation and/or evaluation of the geological body allows definition of a 3D blast volume within which blasting is to be conducted for extraction of the mineral(s) of interest. Such simulation allows zones rich in waste portion and rich in value portion to be identified and may also enable mapping of the blast volume by metal grade. Optimum use of the blasting methodology may require additional exploration over conventional blasting to assess, as clearly as reasonably economically practicable, location of value and waste portions. Further data and more detailed characterisation of the geological body is required than for conventional blasting. That is, most effective or optimal use of explosives to achieve the required fragmentation distributions (for effective separation) in different parts of the geological body requires more accurate control over explosive charge density, through control over placement pattern of explosive charges, and charge density per blast hole than conventional blasting.

Blasting is then advantageously controlled through a blasting design such that those locations in a blast volume high in waste are fragmented to produce “coarser” material than those locations in the blast volume rich in value portion. A software package particularly useful for ensuring that blasting is conducted to achieve this objective—generally of directing the explosive energy from blasting to those locations where greatest degree of fragmentation is required—is the Hybrid Stress Blasting Model (HSBM) developed by the Bryan Mining and Geology Research Centre at the University of Queensland. Other blast simulation and design packages to achieve such objective may also be adopted. Whichever software package, or other technique, is adopted, a blasting design is generated to achieve the desired fragmentation behaviour of the geological body or ore block.

Blast design may involve various inputs and the use of blasting techniques adapted to the geological body, or ore block, to be mined. Among those inputs may be included selection of appropriate electronic detonation systems, decking and the use of selected explosive products. Decking is a process of creating a gap in the explosive column of a blast hole so that blasting energy can be directed to a specific location. Infill drilling is likely also be necessary to target high explosive energy to those areas with higher metal content.

A starting point for blast model design may involve use of a conventional pattern of blast holes, though with a view to targeting explosive charge density to the rich zones of the geological body to ensure finest fragmentation in these zones. Therefore, explosive charge density may need to differ between different blast holes even though the blast holes are laid out in a conventional pattern. Charge regulation to ensure highest explosive charge density corresponds with rich zones of the geological body and lowest with waste zones is likely to be required. Typically, additional infill blast holes are likely to be required for the blasting method to increase explosive energy distribution in value portion (rich and transition zones) of the geological body. Optimal locations for these infill blast holes may be determined based on assay results identifying the value portions of the geological body. More extensive assay than for conventional blasting is likely to be required to enable effective placement of explosive charges as above described.

Further, as mining proceeds, and more assay and rock property data about the geological body becomes available, the blast model can be adapted using this data to enable efficient control over blasting and achievement of fragmentation distributions which enable more effective separation of mineral stream(s) concentrated in the mineral to be extracted.

The waste portion may still comprise some mineral(s) of economic interest. Therefore, further treatment, for example beneficiation by dry sorting, or processing of the waste portion for liberation of minerals or metals, for example by heap or dump leaching, is not precluded. However, following the method of the present invention, extractive processes will be focused on the value portion, typically comprised in the above described fine stream.

The blasting method of beneficiating the geological body enables reductions in comminution energy consumption and may make exploitation of some ore bodies economic where previously processing could not have been economic. The method also offers scope for simplification of concentration and extraction steps when exploiting a mineral resource.

The blasting method of beneficiating minerals of the present invention may be more fully understood from the following description of a preferred embodiment thereof made with reference to the accompanying figures:

FIG. 1 is a graphic showing a blast design for a geological body to be beneficiated using blasting in accordance with the method of one embodiment of the present invention;

FIG. 2 is a graphic showing generation of fines in an ore body when a method in accordance with one embodiment of the present invention is conducted.

FIG. 3 shows fragment size distributions for ore and waste streams produced following blasting of the geological body with the blast design illustrated in FIG. 1.

Referring to FIG. 1, the graphic shows a geological body or ore block 10 including a value portion in the form of a mineralized ore body—for example being a finely disseminated gold containing ore body 20 containing 5 g/t ore in gold. The ore body 20 forms an incline or vein in geological body 10. Surrounding the gold containing ore body 20 is a body 30 of waste or gangue material, this forming the waste portion of ore block 10. The gold containing ore body 20 was defined following exploration and sampling and it is established that the ore body 20 comprises finely disseminated ore. Such exploration activity also well defines waste body 30, allowing a 3D model of the ore block 10 to be designed using modelling packages available to those skilled in the art. The ore block 10 could not be economically exploited by conventional means.

In addition, the rock material properties of ore body 20 are identified by use of geophysical probes, as are known in the art. Such identification enables the geo-mechanical characteristics of the ore body 20 to be identified and input to the blast design as described below.

These procedures result in identification of a 3D blast volume 40 which is to be blasted during the mining operation. Prior to blasting, a blasting design is developed with the use of a blast simulation software package, here the Hybrid Stress Blasting Model (HSBM) developed by the Bryan Mining and Geology Research Centre at the University of Queensland. The HSBM model is described in literature including S Esen, A Hybrid Stress Blast Model based on Non-Ideal Detonation Behaviour of Commercial Explosives, PhD Thesis, University of Queensland, 2004; and I Onederra et al., Burden Movement Experiments using the Hybrid Stress Blasting Model (HSBM), Explo 2007, Wollongong, 3-4 Sep. 2007, the contents of which are hereby incorporated herein by reference.

The blasting design could involve, as a starting point, setting a pattern of blast holes useful for conventional blasting of ore body 20. though here with a view to targeting explosive charge density within ore body 20 with the object of ensuring finest fragmentation in the ore body 20. Therefore, explosive charge density may need to differ between different blast holes even though the blast holes are laid out in a conventional pattern. Charge regulation to ensure highest explosive charge density corresponds with value portions (rich zones) of the geological body and lowest explosive charge density corresponds with waste portions is required. Therefore, additional infill blast holes are required for the blasting method to increase charge density and explosive energy distribution in identified value portions of the ore body 20. Optimal locations for these infill blast holes are determined based on assay results identifying the value portions of the ore body 20. More extensive assay of ore body 20 than would be required for conventional blasting is likely to be required to enable effective placement of explosive charges as above described.

The blasting design has the objective of generating a higher degree of fragmentation in ore body 20 than in waste body 30. To that end, the blasting design, which is illustrated by FIG. 1, here requires a majority of the explosive charges to be located in ore body or value portion 20. That is, release of explosive energy during blasting is concentrated in value portion 20 and only the minimum blasting energy required for loading and hauling is used in the waste body 30. Various blast designs can be used to achieve this, for example two blast holes, one marked 22 and the other being marked 24, are drilled into the ore body 20. Hole 22 is drilled to a slightly greater depth than blast hole 24. However both holes 22 and 24, which have a diameter of 89 mm, are drilled to near the base of ore body 22. Both blast holes 22 and 24 are then packed with an explosive emulsion in respective locations 22 a and 24 a at the base of blast holes 22 and 24.

A single hole 26 is drilled into the waste body 30 and this hole 26 is packed with explosive emulsion at three locations (one location 26 a being within the ore body 20 and located between locations of explosive 22 a and 24 a, and two locations 26 b and 26 c within the waste body 30.

Schematically, lesser quantities of explosive emulsion are used at locations 26 b and 26 c than at the other locations for explosive. However, the quantity of explosive emulsion is selected to assure a lower degree of fragmentation within waste body 30. This blasting design results in controlled blasting ensuring that material in the ore body 20 is fragmented to a smaller average particle size, D₅₀, than the material in the body of waste or gangue. The resultant focused generation of fines within ore body 20 is shown schematically in FIG. 2 where the clouded region 25 shows where finest fragments are generated. The relative darkness of the clouded region 25 shows the relative degree of fragmentation with the darkest areas of clouded region 25 being the most finely fragmented. Some fines are also formed in the waste body 30, in the vicinity of hole 26 but not to an extent which affects beneficiation in accordance with the blasting method described here. Fragmentation of the waste body 30 is schematically illustrated by the grey lines 36. The resultant fragment size distributions for the value or ore stream; and waste stream are illustrated in FIG. 3.

FIG. 3 clearly shows a distinct difference between D₅₀ for the value or ore stream (80 mm) and D₅₀ for the waste stream (210 mm), this difference being sufficient to allow selective physical separation on the basis of size. For example, screening could be used to separate the value or ore stream, this being a finer fraction from the waste stream which is a coarse stream. The fine ore stream is concentrated and has a higher gold content than the coarse stream. Therefore, as a result of the controlled blasting, a stream having higher metal content is generated than would be generated by conventional blasting which produces material with D₅₀ of 150 mm and still requiring beneficiation. The blasting therefore results in beneficiation or upgrading of ore block 10 not achievable by conventional blasting.

In conventional blasting, in contrast, the entire ore block 10 would be blasted at a given energy to achieve a top size that can be transported in haul trucks and processed through the primary crusher which is not always reliably achieved.

Following mining of ore block 10, run of mine material is screened at an optimised at a cut size of 200 mm, this size being determined as the optimum economic size fraction in this example, with >200 mm coarse material going to a waste dump or leach pad to enable heap or dump leaching. The <200 mm fine material is directed to a process plant for high grade conventional gold extraction,

It will be appreciated that use of controlled blasting, as above described, would not require a change of screen size. Mined material could still be screened using a 150 mm screen size. The undersize material from such screening would have higher grade than material obtained from conventional mining operations. Thus, use of controlled blasting achieves benefits without there being any requirement for re-design of the screening plant.

Table 2 provides results of a simulation showing how the potential uneconomic ore block 10 has been beneficiated by controlled blasting in accordance with the method described here.

TABLE 2 Scenario illustrating the Upgrading of Ore Block 10 Ore Grade Recovered tonnage (t) (g/t) Metal (Oz) Results Conventional Blasting 1514 1.01 491 Uneconomic Controlled Blasting 717 1.91 438 Economic

The grades in Table 2 are less than the 5 g/t gold present in the ore since waste material is still present after blasting. However, because controlled blasting results in a lesser tonnage of material to be processed, the effective grade is very significantly increased over conventional blasting.

Essentially, the grade of the ore feed to the processing plant has been increased by 90 percent and the tonnage to be processed has been reduced by 50 percent. Even though less metal has been recovered than using conventional blasting, the savings in energy, approximately 20 percent, and other capital and operating costs resulting from the lower tonnage processed, an uneconomic ore body 10 has become economically viable to mine and process.

The method is not limited to beneficiation of gold bearing ores. Base metal ores, such as copper and other base metal ores, or ores containing platinum group metals may also be beneficiated in accordance with the method.

It will be apparent that the above described beneficiation method results in increased energy efficiency and reduced processing costs, such cost reduction increasing the possibility for mining and processing lower grade mineral deposits. In particular, comminution costs such as crushing and milling energy costs may be reduced in accordance with the method of the invention. It also highlights to ability to beneficiate previously uneconomic ore bodies by upgrading the metal content through blasting and screening, thereby offering the potential to expand ore resources available for treatment.

Modifications and variations to the blasting method for beneficiating minerals as described in the present specification will be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed to be within the scope of the present disclosure. 

1. A blasting method for beneficiating minerals comprising mining of a geological body including a value portion having at least a threshold content of a mineral to be extracted and a waste portion relatively lean in content of a mineral to be extracted wherein mining comprises controlled blasting of the geological body such that the value portion fragments differently than the waste portion, the difference in fragmentation between the value portion and the waste portion enabling separation of a mineral stream concentrated in the mineral to be extracted from the waste portion.
 2. A method of claim 1 wherein separation of the mineral stream concentrated in the mineral to be extracted is achieved by physical separation.
 3. A method of claim 2 wherein physical separation is separation on the basis of fragment size.
 4. A method of claim 3 wherein physical separation is by screening.
 5. A method of claim 3 wherein blasting produces a fine material and a coarse material, said fine material having a higher concentration of said mineral to be extracted than said coarse material, the fine and coarse fractions being separated by physical separation.
 6. A method of claim 5 wherein said blasting is controlled through a blasting design such that said waste portion of the geological body is fragmented to produce coarser material than material from said value portion of the geological body.
 7. A method of claim 6 wherein blasting design comprises charge regulation to ensure highest explosive charge density corresponds with said value portion of the geological body and lowest explosive charge density corresponds with said waste portion of the geological body.
 8. A method of claim 7 wherein blasting involves use of a conventional pattern of blast holes and additional infill blast holes to increase explosive energy distribution in said value portion of said ore.
 9. A method of claim 2 wherein separation of the mineral stream concentrated in the mineral to be extracted is achieved by sorting, preferably dry sorting.
 10. A method of claim 1 wherein said geological body includes an ore body.
 11. A method of claim 10 wherein said ore body contains at least a base metal or a precious metal. 